Nuclear-spin conversion analysis of ν + ν combination band of crystalline methane in phase II.

We conducted reflection-absorption infrared spectroscopy in an ultrahigh vacuum and observed the ν + ν combination band of crystalline methane in phase II, where the rotating and librating species coexist. We analyzed the time- and temperature-dependence of the spectrum due to the nuclear-spin conversion of methane to update the assignment of the rotational and librational structure of this band. The conversion analysis performed in the present work will also be applicable to the detailed assignments of overtones and other combination bands.

Spectroscopic determination of interconversion rates among three nuclear spin isomers of methane in crystalline II.

We measured infrared absorption spectra of crystalline II of CH and succeeded in detecting a prominent Q(2) peak in the ν vibrational region by rapid cooling after annealing as well as previously reported rovibrational and librational-vibrational peaks. The integral intensities of the R(0), R(1), and Q(2) peaks were found to show biexponential dependence on time. This clearly demonstrates the interconversion among the three nuclear-spin isomers occupying low-lying rotational levels.

Infrared spectroscopic investigation of nuclear spin conversion in solid CH4.

Infrared spectra of solid CH4 were studied in the ν3 and ν4 vibrational regions. The phase I crystal around 30 K showed broad absorption bands, whereas the phase II crystal at 6.9-10.